Grass different shades of green

hi all, see picture attached we have existing grass in the middle, but turf laid either side which has been down a few months now the colour is evident between the two, the new turf is growing well at rapid speed actually our old grass isn’t… it’s patchy discoloured not quite sure what we need to do here do we rip it up and prepare the ground for turf like we have done either side - is there any chance the colour will change over time? I was thinking of getting green thumb or someone who treats lawns into help but if this doesn’t help the two lawns look indifferent I don’t know what will? Thanks in advance for your help.